OCG Rulings

When "Skill Drain" applies its effect, the ATK of a "Number 17: Leviathan Dragon" that was increased by its own effect returns to its original value. Afterwards, even if the effect of "Skill Drain" is no longer applied, the ATK remain at its original value. Furthermore, the effect of being unable to attack the opponent directly while this card has no Xyz Materials is also negated, and this card is able to attack the opponent directly.[2]

References

↑Konami OCG FAQ: While "Skill Drain" or "Effect Veiler" apply their effects, are the effects of "Number 17: Leviathan Dragon" of increasing its own ATK by 500 or being unable to attack if it has no Xyz Materials negated?